In the sixth year of the bull run, the U.S. large cap market has had its ups and downs. The S&P 500 peaked at 2,134.7 in the early summer months and promptly collapsed to 1,867 points during the August flash crash.

Today it’s back in black, but only trading just over 1% higher than it started the year.

The only reason that has made this possible is the legendary performance of four tech stocks: Facebook, Amazon, Netflix and Google (now called Alphabet Inc). Together, the FANG stocks have created an impressive $440 billion in market capitalization since January.

For comparison’s sake, that’s over two-thirds the size of Apple’s current market cap.

The FANG stocks comprised just over 3.5% of the weight of the S&P 500 index at the beginning of the year, and now they make up 5.1%. They’ve carried the market and without them the S&P 500 would surely be in negative territory today.

Looking at these companies individually, probably Amazon (AMZN) has been the most impressive over the course of the year. While it didn’t shoot up the 143% that Netflix (NFLX) did, Amazon had a similar performance despite being over six times the size of Netflix. Amazon is now bigger than Facebook in terms of market cap and achieved a gain of 116% through the year.

The only problem is that it is now the most expensive stock on the index, at a seemingly ludicrous P/E ratio of 966. The other stocks are expensive as well: Netflix and Facebook are trading at 329 and 108 times earnings respectively. Google is the lone palatable company from that perspective, trading at only 35 times earnings.

This raises the question of how long the FANG stocks can carry the load and whether their work is done.